The present invention relates to a process for inhibiting corrosion of metal surfaces and minimizing deposits in an air preheater system. More specifically, the present invention relates to a process in which a corrosion inhibiting and antifouling composition is supplied to the air preheater system.
The combustion gases of fuel fired vessels contain various contaminants or combustion by-products which can cause corrosion of system metals when condensed in the cold end of the system, i.e. when the temperature drops below the dew point. Moreover, solids can deposit which can also cause gas flow restrictions in the air heater.
Typical corrosive agents are H.sub.2 CO.sub.3 produced in the combustion of carbon and H.sub.2 SO.sub.4 from SO.sub.3 produced in the combustion of sulfur. As the flue gases pass through the boiler heat exchange system, heat is extracted with consequent cooling occurring. As the dew point of the gas is reached, condensing of these corrosive agents into an aqueous solution occurs. The condensed weak and strong acids corrode boiler system metals-typically, economizers, air heaters, ducts, electrostatic precipitators, stacks, etc. The wet, sticky mass which accumulates in itself is a foulant but also causes entrapment or adhesion of particulates from the flue gas. The corrosion and fouling which occur affect the reliability of the fuel fired unit, reduce efficiency of heat transfer in the energy recovery equipment, cause forced outages, reduce the megawatt output of the unit (deration), increase maintenance costs, and reduce equipment life.
Various techniques, have been employed in attempt to prevent corrosion and inhibit deposition in air preheater systems, i.e., at the cold end of boiler combustion systems. One method maintains the exit gas temperature above the dew point. This method, however, causes loss of usable energy.
Another method involves disassembling of the cold end components, cleaning them of deposits and replacing deteriorated materials as required. This method also causes loss of usable energy due to fouling. In addition, there is considerable down time and the equipment life is reduced.
In another process inorganic powders such as magnesium oxide, sodium aluminate and the like are added, for example, to flue gases from a boiler. These inorganic powders react with the acidic materials in the flue gases, which are primarily H.sub.2 CO.sub.3 and H.sub.2 SO.sub.4 from the carbon dioxide, sulfur dioxide and sulfur trioxide in the flue gases. These powders are added in proportion to the concentration of such materials to cause a neutralization reaction in which the H.sub.2 CO.sub.3 and H.sub.2 SO.sub.4 are removed as solid materials.
The processes employing inorganic powders, however, have a number of disadvantages. They employ a stoichiometric amount of the powder relative to the amount of the acid material in the gas stream. Also, the reaction product between the acid in the gas stream and the inorganic powder is a solid which can cause fouling of the air preheater, especially when the air heater metal temperature is below the water dew point. Moreover, the inorganic powders are not distributed very well over the air preheater surfaces, because they are powders. In addition, these powders require chemical powder feed equipment, which is costly to purchase and install and to maintain. Still further, the inorganic powders increase the amount of particulate matter that a downstream particulate collector must remove.
Others have employed neutralizing amines as cold-end additives. For example, U.S. Pat. Nos. 4,134,728, 4,134,729, 4,185,071 and 4,206,172 disclose the use of alkanolamines and ethylene polyamines as such cold-end additives, either alone or in combination with each other or in combination with sodium aluminate. The effectiveness of these amines as corrosion inhibitors are, however, relatively limited. The amount of amine added is directly related to the acid content of the combustion stream to be treated and therefore requires a relatively large amount of amine for such processes. Moreover, the amines provide relatively unstable formulations with water and therefore cannot be stored over a wide range of temperatures over long periods of time without difficulty. The amines typically form dispersions in water.